PROJECT SUMMARY/ABSTRACT BACKGROUND AND CENTRAL HYPOTHESIS: Ribosomopathies are caused by disruptions in making ribosomes, the intricate molecular machines responsible for synthesizing all cellular proteins. While one might expect defects in such an essential process to result in an inviable organism, such disorders instead manifest in tissue-specific signs and symptoms. In the ribosomopathy Treacher Collins syndrome, this tissue specificity manifests in craniofacial defects arising from the nucleolar stress response in stress-sensitive neural crest cells. Mutations in the Paired box 9 (PAX9) gene result in the loss of more than six permanent teeth, or oligodontia, as well as craniofacial dysmorphology and hair loss. These symptoms are similar to those of the ribosomopathy, Treacher Collins syndrome. A genome-wide siRNA screen in human MCF10A cells for proteins required for the production of ribosomes revealed an unexpected role for the RNA Polymerase II (POL II) transcription factor, PAX9, in human ribosome biogenesis. Preliminary results have shown that PAX9 depletion results in defects in pre-ribosomal RNA (pre-rRNA) processing as well as in decreased transcription of a subset of genes that encode nucleolar proteins. I hypothesize that PAX9 alters mammalian ribosome biogenesis indirectly by promoting the transcription of one or more of these genes required for making ribosomes in the cell nucleolus. I propose that the craniofacial dysmorphology and oligodontia seen in humans with PAX9 mutations is connected to PAX9?s crucial role in ribosome biogenesis. SPECIFIC AIMS: Specific Aim 1 will confirm that the PAX9 protein is required for the transcription of candidate genes encoding nucleolar proteins. I will confirm that the northern blot phenotype is directly related to PAX9?s transcription of the candidates. I will also demonstrate that PAX9 binds directly to and regulates the transcription of the candidate genes. Specific Aim 2 will test the extent to which PAX9 depletion or mutation in a developing embryo connects craniofacial dysmorphology to ribosome biogenesis. First, I will establish Xenopus tropicalis as a model system for studying the effects of PAX9 on both craniofacial development and ribosome biogenesis. I will then test the extent to which the candidate genes from Specific Aim 1 recapitulate the defects seen upon PAX9 depletion.